This invention relates in general to the placement of electronic components on printed circuit boards and, more specifically to a method and apparatus for very precisely forming very small solder balls on an array of contact pads on an integrated circuit component substrate.
Originally, printed circuit boards included plated holes into which leads of electronic components were inserted and soldered. Currently, circuit boards tend to have closely spaced surface pads with the component simply placed on the board with leads in contact with the pads, to which the leads are then soldered.
The pattern of closely spaced wires and contact pads for connection to component leads are usually formed on the printed circuit board base by silkscreening techniques. The pattern is typically in the form of a single square or rectangular row, conforming to the leads extending from the peripheral edge of an electronic component. After board fabrication and during assembly operations, a thin stencil having holes conforming to the pad pattern is placed over the pad array and precisely aligned therewith. A solder paste made up of very small solder balls in a slurry of flux and other ingredients is wiped over the stencil with a squeegee, forcing small amounts of paste through the holes onto the pads. The stencil is lifted away leaving a coating of the paste on the pads. The components must then be very precisely placed on the board with the component leads aligned with the pads. Accuracies on the order of 0.0005 inch are often required. Once the component is placed, the solder is melted, generally in an infrared or convection oven, and solder bonds are formed between the pad and lead.
Very large, very expensive, robotic machines have been developed for accurately placing components on such surface mount boards with leads precisely aligned and soldering the leads to the pads in very high volume manufacturing operations. These machines make use of extremely expensive vision alignment equipment and other optical devices to precisely locate the components. These large and complex machines require considerable operator training and the component and board designs cannot be rapidly and easily changed. Exemplary of such machines is that described by Takahashi et al. in U.S. Pat. No. 4,292,116. Such large devices are not economically feasible, or well adapted to, low to medium volume operations where only a few fine pitch parts are placed on relatively few boards or for rework of components removed from such boards.
Many integrated circuit components have a row of very closely spaced contacts around the component periphery. Today, components are being developed that have an array of copper pads across the underside of the components in rows and columns at various pitch locations. Bumps or balls of solder are provided on the copper pads to make interconnect points (a ball grid array) for attachment of the component to a corresponding pattern on a circuit board.
Typically, solder balls are attached to the component pads by screening a flux in paste or gel form onto the underside of the component. Then preformed balls are shaken into a stencil which is aligned with the component pads. The balls are held in place by the gel as the stencil is removed. The assembly is sent through an oven to melt the balls which attach to the copper pad on the substrate, completing the integrated circuit package. This is an effective, although very slow, process.
The component can be fastened to a printed circuit board by normal surface mount processes. A flux paste is applied to the corresponding array of copper pads on the board, the component is placed over the pads with the solder ball array aligned, and the assembly is heated. The solder ball combines with the paste and the sphere, although growing larger, will collapse onto the pads under the weight of the component, making the interconnect.
While good connections are made, in some cases differences in the coefficient of thermal expansion of component and board are enough different to cause high stress at the corners of the array so that connections may fail during thermal cycling in use.
Sometimes ball grid array components must be removed from a printed circuit board for rework of the overall board. Because of the density of the component connections, the components are usually quite expensive, so that the ability to repair and re-use them is important. Desoldering tools and heat profiles have been developed for removing components from a printed circuit board that will retain most, but generally not all, high temperature balls on the component pads. With low temperature balls that melted during mounting the entire component must be reballed. In some cases, solder bridges will form between pads so that solder will have to be completely wiped or wicked off of the pads.
Reballing is often done with small tools used in a manner that essentially mimics the original manufacturing method of balling parts. A first screen is positioned over the contact pad array, a flux past or solder paste is wiped over the screen to add flux to the pad array, then the screen is removed. A second screen with larger holes is placed over the part and pre-formed balls are gravity loaded through the stencil with any excess balls shaken off. The second screen is removed and part is reflowed upside down in an oven. This is a very time consuming process and requires considerable skill.
Thus, there is a continuing need for improved methods and apparatus for installing balls on such components, which is more reliable, takes less time and is more easily used by less highly skilled workers.